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Atrazine, pigweed issues on the minds of corn growers

• Atrazine is under the microscope again.
• Concerns also have arisen about atrazine resistance.
• In recent years, some growers have asked for recommendations on controlling Roundup Ready corn in a replant situation.

Atrazine worries and resistant pigweed problems are just a couple of the weed control issues on the minds of corn producers as they plant the 2011 crop.

“Atrazine is under the microscope again,” says Eric Prostko, University of Georgia weed scientist. “In 2009, EPA called for a new evaluation of atrazine, and that process is continuing today. There are some people who would love to take atrazine away from corn growers. If that ever happens, it would be devastating because it is the foundation of our weed control program in corn.”

Concerns also have arisen about atrazine resistance, says Prostko.

“A few years ago, I reported that we were finding instances of resistance to atrazine, specifically in Macon County, Ga., in areas that were predominately dairy where they were using quite of bit of atrazine in their rotations. We continue to survey in that area. We’ve found multiple resistance in some areas of Macon County, where there is resistance to glyphosate, the ALS herbicides and atrazine,” he says.

In recent years, says Prostko, some growers have asked for recommendations on controlling Roundup Ready corn in a replant situation.

“In the early spring, if we get a late frost, we get a lot of calls about replanting Roundup Ready corn. There are a few things you can do for controlling Roundup Ready corn in a replant situation. In trials, we made an application of SelectMax in one field and Gramoxone Inteon plus atrazine in another. Select Max has been shown to be one of our better treatments if you do have to replant in Roundup Ready corn,” he says.

One change growers may see this year is a change in how the herbicide Accent is marketed, he says. “Historically in Georgia, we have been a big user of Accent, and it has been good for the control of Texas millet or buffalo grass. At the corporate level, DuPont is positioning away from the Accent products and is developing a line of products called the Q Products. Whenever you see a Dupont product with the ‘Q’ in it, it means it contains a safener called isoxadifen.

“Instead of Accent, they’ll be promoting Steadfast Q. This is a combination product that contains Accent. With Steadfast, we’d use 1.5 ounces. At that rate, we don’t get as much of the Accent formulation in the treatment as we would with a regular application of Accent, which is two-thirds ounce. But after several years of testing the product, I don’t think we lose anything going from Accent to Steadfast Q.”

Many Georgia growers, says Prostko, have turned to LibertyLink crops in response to problems with glyphosate-resistant Palmer amaranth pigweed.

“Glufosinate or Ignite does give us another mode of action to control pigweed if you grow cotton. About 22 percent of our cotton this past year was treated with glufosinate. We can use it in LibertyLink corn and it can be effective against Palmer amaranth if treated in a timely fashion. You probably still want to tank-mix it with atrazine. It’s also good on other weeds such as Texas millet, morningglory and sicklepod.”

But, he adds, there are a few concerns with the use of Ignite. “We’re seeing in our research that you still need a residual herbicide in all crops that we’re growing. Depending on the density of the Palmer amaranth, one residual may not be enough. We may need two to give us that extended residual control.”

Protect the technology

With LibertyLink cotton, corn and soybeans all now available, growers need to be careful and protect the technology, he says.

Resistant Palmer amaranth pigweed is pretty much spreading throughout the Southeast region, says Prostko. As for control strategies, growers should first start with a clean field, he says.

“We’re lucky in Georgia that 65 percent of our corn crop is irrigated, and we can use irrigation to activate residual herbicides. Also, if you haven’t been tilling in awhile, you’ll certainly see the benefit of a moldboard plow, maybe not every year, but once every three to four years. Using extreme cover crops, residual herbicides and timely postemergence applications are other keys to controlling resistant pigweed, he says.

Herbicide programs are available, says Prostko, for controlling Palmer amaranth in field corn.

“As far as residuals, we still rely on atrazine. We can use Dual Magnum in areas where atrazine resistance might be a problem, and we can use Micro-Tech. In Roundup Ready corn, we’ve seen success with glyphosate, atrazine and Prowl; Expert, which is a three-way mix of atrazine, glyphosate and Dual; and Halex GT, which is a three-way mix of glyphosate, Dual and Callisto.

“For LibertyLink corn, Ignite and atrazine is our foundation. Whether or not we mix in Prowl depends on if we’re having problems with Texas millet. For conventional growers, some programs that have looked good have included Steadfast Q and atrazine and new products like Laudis and Callista.”

Also, it’s important to manage post-harvest populations of Palmer amaranth, he says. “We get our corn off in late July, early August, and don’t get a frost until mid-November. So that’s a long time frame when we let these populations produce seed. In the fall, they can go from seed to seed in 35 days. You might have done a great job in corn all year, but if you allow these populations to go to seed, we’re back to ground zero.

“We can mow or till, and we can use something like paraquat and 2,4-D. There are a lot of control strategies, with the goal being to prevent those plants from producing seed.”

With resistance becoming more of an issue, Prostko says growers might want to consider some of the products that have been introduced in recent years. These can include products like Callisto, Laudis and Status. Callisto is a postemergence herbicide with a unique “bleaching” mode of action. Laudis is another product with a “bleacher” mode of action, while Status contains dicamba and is used for broadleaf weed control.

Looking to the future, Prostko says Dow Agroscience will be releasing DHT corn with enhanced tolerance to 2,4-D and products like Assure or Fusilade. It’ll also be stacked with glyphosate. “New herbicides include Zidua, which probably will come out next year. It’s a grass herbicide. The other one is Capreno, which is labeled right now.”